1. Field of the Invention
The invention of the present application relates to a stator core.
2. Description of the Related Art
A rotating electrical device such as a motor or a generator is provided with a stator that generates rotating magnetic field to rotate a rotor. The stator includes an approximately cylindrical stator core and a coil, and the coil is assembled to a tooth (pole tooth) that is disposed in the stator core. A plurality of teeth are disposed in an inner circumference surface of the stator core along its circumferential direction, and a gap between adjacent teeth is referred to as a slot. In order to improve density of the coil (ratio of volume of the coil occupying a space of the slot) in the slot, a tooth that is disclosed in Japanese Patent Application Publication No. 2008-160939 and referred to as a trapezoidal tooth is used, for example. The trapezoidal tooth has a trapezoidal cross section that is taken vertically along a central axis C of a stator core 110 as shown in FIG. 8, and the tooth is formed so as to have a narrower width in a direction toward the central axis C of the stator core 110.
A trapezoidal coil 114 that follows the shape of the trapezoidal tooth 112 is assembled to the trapezoidal tooth 112. When the cross section of the tooth and the coil has the trapezoid shape, the density of the coil in the slot 116 improves in comparison with a case where the cross section of the tooth and the coil has a rectangular shape as shown in FIG. 9. The fact described above shows that the rotating electrical device can be downsized while maintaining the output by using the trapezoidal tooth 112.
As shown in FIG. 10, when a trapezoidal coil 122 is assembled to a designated trapezoidal tooth 112 in a case where a trapezoidal coil 114 has already been assembled to adjacent tooth 112, the trapezoidal coil 122 cannot be assembled to the designated trapezoidal tooth 112. In order to facilitate the assembly of the coil, at least one rectangular tooth 118 is disposed in the stator core 110 other than the trapezoidal teeth 112 as shown in FIG. 11. As shown in FIG. 11, even if the trapezoidal coils 114 are assembled to the adjacent trapezoidal teeth 112, a rectangular coil 120 can be assembled to the rectangular tooth 118. In the assembly of the coils to the teeth, the trapezoidal coil 114 is first assembled to the trapezoidal tooth 112 adjacent to the rectangular tooth 118, and then the trapezoidal coils 114 are sequentially assembled to the trapezoidal teeth 112 along the circumferential direction of the stator core 110, and finally the rectangular coil 120 is assembled to the rectangular tooth 118.
Here, the tooth is subjected to magnetic flux from a permanent magnet 126 that is provided in a rotor 124. FIG. 12 shows a schematic view in which the magnetic flux produced by the permanent magnet 126 flows into the tooth. The magnetic flux principally flows from a top surface 128 of the tooth into the tooth, but part of the magnetic flux flows from a side surface 132 of the tooth into the tooth as leakage flux 130. This is because magnetic flux density increases to be saturated in the process of flowing from the permanent magnet 126 into the tooth, and thus the magnetic flux that departs from the top surface 128 of the tooth is produced.
The intensity of the leakage flux 130 differs between a case where the tooth is the trapezoidal tooth 112 and a case where the tooth is the rectangular tooth 118. That is, when a magnetic path from the rotor 124 to the side surface 134 of the trapezoidal tooth 112 (shown with an alternate long and short dash line in FIG. 12) and a magnetic path from the rotor 124 to the side surface 132 of the rectangular tooth 118 are compared, the magnetic path of the rectangular tooth 118 that passes through atmosphere is longer than the magnetic path of the trapezoidal tooth by the length that is indicated with a symbol Δd in FIG. 12. Because the atmosphere has higher magnetic reluctance than metal that is a material of the tooth, the magnetic flux flows through a long atmospheric passage, and therefore the intensity of the magnetic flux is reduced. As a result, total amount of the magnetic flux that flows into the rectangular tooth 118 is smaller than the total amount of the magnetic flux that flows into the trapezoidal tooth 112.
If the amount of magnetic flux varies due to the shape of the tooth, eccentric force in a radial direction is produced on the rotor 124. The phenomenon will be described with reference to FIG. 13. The rotating electrical device shown in FIG. 13 is a three-phase type alternating-current rotating electrical device, and the rectangular tooth 118 is allocated to a V-phase. When the magnetic flux flows from the permanent magnets 126 of the rotor 124 to the V-phase tooth, if the amount of magnetic flux varies due to the shape of the tooth, electromagnetic force produced on the rectangular tooth 118 and electromagnetic force produced on the trapezoidal tooth 112 facing to the rectangular tooth 118 are not canceled out, and therefore eccentric force 136 is produced in a direction opposite to the rectangular tooth 118. Due to the eccentric force 136, the rotor 124 and a shaft 138 of the rotor 124 are pulled in the direction opposite to the rectangular tooth 118. Thus, rotation of the rotor 124 becomes unstable, noise and vibration characteristics (NV characteristics) of the rotating electrical device deteriorates.